Optical signals in standard, non-polarization preserving optical fibre-based communication systems experience random changes in polarization state from one end of the fibre to the other due to fibre birefringence induced by temperature fluctuations and physical stresses on the fibres. Random polarization changes are evidenced at the output end as polarization dependent loss (PDL) and in some instances polarization mode dispersion (PMD).
In order to correct the polarization state of lightwave signals emerging from the optical fibre transformers have been developed to transform the fibre output polarization into the prescribed polarization state for applications such as heterodyne detection and interferometric signal processing. Conventional polarization transformers provide compensation but require a reset cycle when their operating range is exceeded. Unfortunately, reset cycles give rise to periods of unacceptable data loss. Endless polarization transformers provide continuous control of the polarization state over an infinite range of polarization compensation.
Endless polarization transformers have been developed using cascaded polarization transformers having a limited transformation range such as fibre squeezers and electrooptic devices using lithium niobate or PLZT. While these cascaded devices permit truly endless (reset free) operation, individual elements within the devices still require occasional reset cycles. Although the reset cycles can be performed without affecting the overall polarization transformation (quasi-endless polarization control), these devices generally fail to permit polarization control during reset cycles. Moreover, they require sophisticated and even computer controlled drive algorithms for proper operation.
Fibre squeezers mechanically induce birefringence in the fibre axes to cause retardation between the two orthogonal modes perpendicular and parallel to the direction of pressure. U.S. Pat. No. 5,561,726 in the name of Yao, describes a system that utilizes a rotatable fibre clamp to supply the necessary retardation and optical axis orientation. Although this device can be used for fixed wavelength and temperature and polarization it cannot be used to control real time polarization fluctuation in transmission fibres, because it requires mechanical movement for its control.
Recently, a reset-free, endless polarization transformer was demonstrated performing general polarization transformations from any arbitrarily varying optical input polarization into any arbitrarily output polarization by producing adjustable elliptical birefringence of constant total phase retardation in a single-mode waveguide. See U.S. Pat. No. 4,966,431 issued to Heismann on Oct. 30, 1990. A particular transformation is obtained by adjusting the azimuth of linear birefringence and the ratio of linear to circular birefringence. In its integrated-optic realization, the endless polarization transformer includes at least one cascadable transformer section comprising cascaded first and second TE TM mode converters. Phase shifting (TE/TM) is performed in a section between the mode converters, in a section following the mode converters, or both between and following the mode converters. All sections are formed over a birefringent waveguide capable of supporting propagation of TE and TM optical signal modes. While the recent endless, reset-free polarization transformer is cascadable and affords simplicity of design and operation over prior art devices, it cannot be overlooked that this polarization transformer has a relatively narrow optical bandwidth at wavelengths of interest less than 1 nm at 1.55 ..mu.m and permits only limited tunability over a small wavelength range approximately 10 nm.
Heismann in U.S. Pat. No. 5,212,743 entitled Automatic Polarization Controller Having Broadband Reset-Free Operation, incorporated herein by reference discloses a wide optical bandwidth and broad wavelength tuning range achieved in a reset-free, optical, automatic polarization controller by combining three controllable fractional wave elements in cascade and further by controlling the orientations of both outermost fractional wave elements to differ by a prescribed angular amount which is maintained substantially constant. Synchronous control of both outermost fractional wave elements maintains the prescribed angular difference may be maintained constant during operation of the polarization controller.
In the embodiments described by Heismann, the three fractional wave elements are provided in the form of an endlessly rotatable half-wave element and two synchronously rotatable quarter-wave elements wherein the half-wave element is placed between the quarter-wave elements. Each fractional wave element varies the orientation of retardance along its optical wavepath and introduces a specified phase retardation. Embodiments of the polarization controller are realized using either distributed bulk optic devices or integrated electro-optic waveguide devices. Rotation of the elements is afforded by a feedback control circuit which monitors the output optical polarization and derives appropriate electrical drive signals to achieve the proper rotation of the elements. Although the device taught by Heismann appears to achieve its intended function in many instances it does not provide suitable wavelength and temperature independence.
It is an object of this invention to provide an automatic polarization controller having broadband operation wherein undesired reset operations are obviated and which is relatively wavelength and temperature independent.
It is a further object of the invention to provide an inexpensive, highly responsive device for controlling polarization of an input beam of light having varying polarization states.